In recent years, attempts have been made in various fields to use the functionality of coating films formed by preparing slurries, pastes or the like, which contain functional materials and will hereinafter be referred to as “slurries”, into coating formulations and applying the coating formulations.
For example, a paste-form, conductive coating formulation composed of a conductive filler, binder resin, curing agent, solvent and the like is used as a conductive adhesive, conductive paint, conductive ink or the like depending on its application (Non-patent Document 1). A coated, magnetic recording medium such as an audio tape, video tape, floppy disk or the like is manufactured by applying, onto a base film of a polyester or the like, a magnetic coating formulation with magnetic particles of submicron size uniformly dispersed in a polymer solution. Further, each electrode structure of a lithium ion secondary cell is realized by mixing a binder with an active material and conductive aid to prepare a slurry, coating the slurry onto a collector foil, and then drying it (Non-patent Document 2).
As common attributes that allow each of the above-described various coating formulations to fully exhibit its functionality, the dispersoid is uniformly dispersed in the dispersion medium, and moreover, the coating film to be formed can realize high adhesiveness. In other words, for allowing a functional filler to fully exhibit its functionality by using a slurry with the functional filler contained therein, it is essential conditions that the state of the slurry is appropriate for the exhibition of the functionality of the filler, specifically that the filler is uniformly and stably dispersed and can form a coating film of high adhesiveness Choosing adequate solvents with a focus being centered around the dispersibility of a filler with a view to meeting such conditions, nonaqueous (organic-solvent-based) solvents (dispersion media), which are excellent in the uniform dispersibility of the filler, show high adhesive force and can be readily dried, are overwhelmingly advantageous as solvents (dispersion media), and as a matter of fact, have been widely used.
However, organic solvents are not only volatile and high in environmental load but are also required to take genotoxicity into consideration, and therefore, still involve problems in safety and workability. In recent years, there is an increasing concern about the protection of environment and the prevention of health hazards in many industrial fields, leading to increasing demands toward VOC reductions, solventless coating and the like in connection with the use of organic solvents involving such problems as described above. It is, accordingly, required to switch to products that are friendly to the environment and people.
Now, those which are attracting the greatest attention as products friendly to the environment and people are water-based products or products made from raw materials of biological origin, which are expected to become part of solventless or post-petroleum products. Various problems, however, arise if water is used as a solvent in place of an organic solvent in a slurry that contains a conductive carbon filler. In a water-based slurry, for example, filler particles tend to agglomerate in the slurry when they are in a charged state, and moreover, they are prone to settling due to a large difference in specific gravity between the solvent and the solute, thereby raising a problem in that their uniform dispersion is very difficult. In addition, it is not easy to find raw materials of biological origin, which exhibit film-forming ability and dispersing ability and can replace conventional raw materials of petroleum origin.
As general measures to cope with dispersion failures, addition of a dispersant, surface treatment, microencapsulation or ultrasonic treatment of a filler, introduction of polar groups into a polymer, and the like can be contemplated. In fact, examples of the addition of a dispersant include the attempt to use a water-soluble, amphoteric dispersant for a slurry composition that contains a microparticulated black inorganic oxide useful in paints, inks, rubbers and plastics, electronic materials and the like (Patent Document 1) and the attempt to use a compound having one or more basic functional groups in an composition for cells, which contains a conductive aid (Patent Document 2). Further, examples of the surface treatment of a filler include various proposals such as the attempt to form surface treatment layers by reacting the metal oxide on surfaces of a microparticulate metal-oxide filler with a hydrophilic silane coupling agent (Patent Document 3). In addition, proposals have been made including the application of ultrasonic vibrations to a paste, which contains an inorganic oxide filler, to disperse the filler and the formation of an insulating resin on surfaces of a conductive filler to provide microencapsulated conductive filler.
However, the dispersion media used in these proposals are primarily organic solvents, and water-based media are used only in a very small number of cases. In contrast to these proposals, it is strongly desired, from an increasing concern about the protection of environment and the prevention of health hazards in recent years, to develop a method that uses a water-based slurry, which is friendly to the environment, is low cost and is high in safety, and that can uniformly disperse a filler.
Upon attempting the dispersion and stabilization of a filler in a water-based slurry, the use of the above-described, respective methods may be contemplated. Among them, the use of a dispersant is advantageous when the simplification of the production process and coating system and the cost matter are taken into account. As a dispersant for use in a water-based slurry, it is possible to mention a polycarboxylate salt or phosphate amine salt used in the field of paints (Non-patent Document 3), a polyacrylamide as a high-molecular dispersant (Non-patent Document 4), or the like. When a reduction in environmental load is taken into consideration, however, preferred is a substance of natural origin rather than a petroleum-based substance. In this regard, a proposal has been made about the use of carboxymethylcellulose as a water-based dispersant upon production of each electrode for a nonaqueous secondary cell (Patent Document 4). According to a study by the present inventors, however, there is still a room for an improvement in its dispersing effect. On the other hand, the use of a petroleum-based binder resin is needed to form a strong coating film. There is, accordingly, an outstanding desire for a binder resin that despite of a substance of biological origin, can exhibit adhesiveness which is by no means inferior to that available from a petroleum-based binder resin.
As an expected application for the above-described water-based slurry composition, a coating formulation for electrode plates in electricity storage devices such as secondary cells or capacitors is considered. The demand for these electricity storage devices has been significantly growing in recent years. Each electrode plate gives considerable effects on the performance of an electricity storage device, and is an electrode member with unit members such as an electrode layer and collector integrated therein. Concerning such an electrode plate, proposals have been made to permit its production in the form of a thinner film with larger area such that it can be provided with an extended charge-discharge cycle life and an increased energy density. As to lithium ion cells, for example, Patent Document 5, Patent Document 6, etc. disclose positive electrode plates each of which is obtained by dispersing or dissolving a conductive material and binder along with powder of a positive-electrode active material such as a metal oxide, sulfide or halogenide in an appropriate solvent to prepare a paste-form coating formulation, providing as a substrate a collector formed of a foil of a metal such as aluminum, and applying the coating formulation onto a surface of the substrate to form a coating film layer.
A capacitor, which makes use of an electric double layer formed at an interface between a polarizable electrode plate and an electrolyte, is used as a memory backup power supply, and its use in fields that require large outputs like a power source for an electric car is also attracting interests. For large outputs, this capacitor is hence required to have both a high capacitance and a low internal resistance. Like a negative electrode plate for the above-described cell, the electrode plate for the capacitor is produced by applying onto a collector a coating formulation, which is generally formed of a binder, conductive material and the like mixed together, and then drying the coating formulation.
As a resin binder for use in the above-described coating formulation for the electrode plates in the above-described electricity storage device such as the lithium ion cell or capacitor, a fluorinated resin such as polyfluorinated vinylidene or a silicone-acrylic copolymer is used, for example. A negative electrode plate (cell) or polarizable electrode plate (capacitor) is obtained by adding a solution of a binder in a suitable solvent to an active material such as a carbonaceous material to prepare a paste-form coating formulation and then applying the coating formulation onto a collector. In the above-described coated electrode plate, the binder employed to prepare the coating formulation is required to be electrochemically stable to a nonaqueous electrolyte and to be free from dissolution into the electrolyte of the cell or capacitor, to remain free from substantial swelling by the electrolyte, and further to be soluble in a certain solvent to permit the coating.
On the other hand, it is practiced to form a protective film on a surface of a metal material such as aluminum, as a base metal material of a collector, by coating a solution of one of various resins. The resulting film is excellent in the adhesiveness to the metal surface, but is accompanied by a problem in that its durability to an organic solvent is insufficient.
In the electrode plate for the cell or capacitor, said electrode plate being obtained by applying the above-described coating formulation onto the surface of an aluminum foil, copper foil or the like as the collector, the coating film layer formed by the coating and drying is accompanied by problems in that its adhesiveness to the collector and its flexibility are insufficient, its contact resistance to the collector is high, and peeling, flaking, cracking and/or the like of the coating film layer takes place during assembly steps of the cell or capacitor or upon charging and discharging the same.
As described above, the conventional cell or capacitor is accompanied by the problems of the poor adhesion between the electrode layer and the collector (substrate) and the high internal resistance at the interface between the electrode layer and the substrate. A variety of coating formulations have been proposed to solve these problems. Coating film layers formed with these coating formulations lessen the adhesiveness problem, but make still higher the resistance between the electrode layer and the collector. Therefore, none of these coating formulations have led to a solution to the problems yet. In recent years, there is also an increasing demand for the manufacture of the above-described electricity storage devices such as lithium ion cells and electric double-layer capacitors and their related products with due consideration being paid to the environment. There is hence a demand for a coating formulation and electricity storage device making use of components, materials and a preparation/production method, which are low in environmental load.